(a) Field of the Invention
In the packaging of snack foods it is common practice to employ an outer coated metallized surface to provide both aesthetic and functional properties to the package. The aesthetic properties include an attractively printed package on a normally shiny metallized background whereas the functional properties include the blockage of light transmission which causes deterioration of such foods. It is also desirable to have the appearance of a white-opaque color on the inside of the package, e.g., the inside of a bag of potato chips. The white-opaque color suggests cleanliness and also masks or minimizes the appearance of oil from snack foods.
(b) Discussion of the Prior Art
Alpha-olefin polymeric films have been produced using a cavitated/expanded core in order to achieve opacity. However, the cavitated/expanded cores adversely affect the tensile strength and other physical properties of the film. Such films have also used fillers in the core to achieve opacity. Again, the physical properties of the core are adversely affected and it generally requires very substantial quantities of filler to achieve opacity when opacity is due solely to the use of fillers. The literature relating to such opaque films also recite, a layer or layers which are optionally: metallized; printed on one side in a white color; and the use of fillers. However, none of the prior art discloses the specific combination of this invention. The following references are illustrative of the prior art.
1. U.S. Pat. No. 5,091,236 of Feb. 25, 1992 to L. Keller et al. discloses a multilayer opaque, biaxially oriented polymeric film. The film structure includes: (a) a core layer having voids which cause a significant degree of opacity; (b) a layer adhering to the core layer including up to about 12% of titanium dioxide: and a titanium dioxide-free non-voided thermoplastic skin layer adhering to the other side of the core layer. The titanium dioxide improves opacity and whiteness of the film.
2. U.S. Pat. No. 4,741,950 of May 03, 1988 to L. Liu et al. discloses an oriented, opaque, alfa-olefin laminate film having a non-blocking first surface, a Smooth lustrous second surface which is intended for further film processing operations such as metallization and an expanded core to provide opacity. The core can optionally also contain additional filler for opacity and the anti-blocking skin layer optionally contains 2% to 8% of titanium dioxide to confer enhanced opacity. Among other shortcomings this patent uses voids in the core which adversely affect the mechanical properties and the anti-blocking layer is not heat sealable.
3. U.S. Pat. No. 4,357,383 of Nov. 02, 1982 to M. Howden et al. discloses a multi-layer metallized film of an alpha-olefin having on at least one surface an adherent layer of random copolymer of ethylene with 0.5 to 15% of 3-6 carbon alpha-olefin and a metallic layer on the surface of the adherent layer remote from the substrate. Among a number of options, this film can also have a heat seal layer on the surface opposite from the metallized layer.
4. U.S. Pat. No. 4,692,380 of Sep. 08, 1987 to D. Reid discloses a metallized, biaxially oriented polypropylene film which exhibits good adhesion between the polypropylene and the metal coating. The core has on at least one surface a propylene-ethylene copolymer which contains no slip agent and which has been subjected to corona treatment.
5. U.S. Pat. No. 4,997,700 of Mar. 05, 1991 to L. Bothe et al. discloses a metallizable, heat sealable, biaxially oriented multilayer film which comprises a polypropylene core layer, a first polyolefin heat-sealable surface layer and a second polyolefin metallizable surface layer. The metallizable layer comprises a propylene-ethylene copolymer containing from 1.2 to 2.8% of ethylene. The heat sealable layer comprises ethylene containing polymers, copolymers of ethylene and propylene or copolymers of propylene and 1-butane as well as other olefinic mixtures.
6. U.S. Pat. No. 5,026,592 of Jun. 25, 1991 to S. Janocha et al. discloses an opaque, biaxially stretched oriented, polyolefinic multilayered film comprising an opaque core layer and two transparent skin layers. The opaqueness is attained by the use of inorganic fillers in the core layer. Titanium dioxide is mentioned along with many different fillers as suitable. Among many different options the top layers can be sealable or non-sealable and metallized or non-metallized.
7. U.S. Pat. No. 4,758,396 of Jul. 19, 1988 to B. Crass et al. discloses biaxially oriented film having a single ply or an opaque core layer of a multiply film composed essentially of polypropylene. The opaque core layer includes 10-40% of filler. Titanium dioxide is mentioned as one of the fillers. In addition to the polypropylene opaque single ply or multiply layered film such film can include functional layers such as heat sealing, cold sealing, adhesion promoting, dye coating, and metallizable layers.
HERCULES WTF 503 is a non-heat sealable film having a white-opaque oriented polypropylene core and a metallizable surface. This product is sold by Hercules, Inc. for the insulation market.
In the invention of this application, an alpha-olefin polymer layer containing a slip agent and titanium dioxide are used to form a heat sealable layer which cooperates with a metallized surface on the opposite side of the core layer to enhance the white coloration of the titanium dioxide containing layer. Preferably, the sealable layer is coextruded on to a bulk core layer of a conventional, non-pigmented, non-cavitated and non-expanded, alpha-olefin of 2 to 4 carbon atoms. Also, there is generally coextruded on the reverse side of the core a skin layer of an alpha-olefin having 2 to 4 carbon atoms suitable for further treatment and metallization.
The present invention has the following four main advantages over the prior art:
1. A significant white-opaque appearance is achieved without the need for cavitation, expansion, or inert filling of the core layer. This also eliminates the need for white color printing of the film. This provides the use of smaller quantities of filler to attain the white-opaque look as compared to fillers in the core, provides better mechanical properties, and is more economical.
2. The white pigmented layer is heat sealable over a wide temperature range in spite of the relatively large percentage concentration of filler in this thin layer.
3. The excellent mechanical properties of conventional polyolefin films, e.g., biaxially oriented isotactic polypropylene homopolymer film, are maintained versus cavitated, expanded, or filled opaque polyolefin film.
4. Thinner gauges of laminated film can be used versus cavitated/expanded, opaque alfa-olefin film thus providing a cost advantage.
The inventive film has: a solid white-opaque appearance; excellent heat sealability and sealing range temperature; good machinability; a low coefficient of friction (COF); excellent gas barrier properties; good metal bond strengths; and excellent tensile strength.